(1). Field of the Invention
This invention relates to a radiographic apparatus having a radiation emitting device for emitting radiation in a cone-shaped beam and a two-dimensional radiation detecting device for detecting transmitted radiation images. The radiation emitting device and radiation detecting device are revolvable about an axis of revolution provided by a straight line extending through an object under examination, for scanning a site of interest of the object. Pixel data for sectional images are put to a reconstruction process based on radiation image detection data outputted from the radiation detecting device in response to an emission of radiation in the cone-shaped beam, and on a reconstruction algorithm. More particularly, the invention relates to a technique for reducing a data processing load resulting from a mechanical displacement occurring between the axis of revolution of the radiation emitting device and radiation detecting device and an axis of radiation emission.
(2). Description of the Related Art
Conventional radiographic apparatus of the type noted above include a C-shaped arm driving X-ray radio-graphic apparatus. As shown in FIG. 1, a known C-shaped arm type X-ray radiographic apparatus includes an X-ray tube (radiation emitting device) 51 for emitting X rays in a cone-shaped beam and a two-dimensional X-ray detector (two-dimensional radiation detecting device) 52 (eg. a flat panel type detector) for detecting transmitted X-ray images. The X-ray tube 51 and X-ray detector 52 are mounted at one end and at the other end of a C-shaped arm 53 to be opposed to each other. When the C-shaped arm 53 is driven, the X-ray tube 51 and X-ray detector 52 are moved along two opposite arcuate tracks about a common center located in a patient M. Synchronously with a movement of X-ray tube 51 on one of the arcuate tracks, the X-ray detector 52 is moved on the other arcuate track while maintaining a fixed distance to the X-ray tube 51. In this way, radiography is performed to carry out an image reconstruction process for creating three-dimensional volume data of a region of interest of the patient M (hereinafter called simply “reconstruction process”).
In the image reconstruction process, the X-ray tube 51 and X-ray detector 52 are driven to acquire data from the site of interest of the patient M in each scan position. These data, after a filtering process, are back-projected to predetermined lattice points of a three-dimensional lattice virtually set to the region of interest of the patient M, thereby generating three-dimensional volume data of the region of interest (see, for example, Japanese Unexamined Patent Publication No. 2002-267622, pages 7 and 8, and FIGS. 9 and 10).
However, the conventional X-ray radiographic apparatus has a problem of an excessive data processing load resulting from a mechanical displacement occurring between an axis of revolution RA of X-ray tube 51 and X-ray detector 52 and an axis of radiation emission XA.
In the conventional apparatus, an algorithm for the reconstruction process is simplified by assuming that the axis of X-ray emission XA always exists on a single plane orthogonal to the axis of revolution RA. However, a mechanical displacement could occur between the axis of X-ray emission XA and the axis of revolution RA, whereby the axis of X-ray emission XA deviates from and inclines relative to the plane on which it should exist.
Assuming that the axis of X-ray emission XA is not inclined relative to the plane, as shown in FIG. 2, only one surface of a three-dimensional lattice point matrix V set to the site of interest is visible when seen from the X-ray tube 51. On the other hand, when the axis of X-ray emission XA is inclined relative to the plane, as shown in FIG. 3, two surfaces of the three-dimensional lattice point matrix V are visible. Specifically, when a reconstruction process is carried out, assuming the state shown in FIG. 2 although the actual state is as shown in FIG. 3, a displacement will occur between each lattice point J in the lattice point matrix V and a corresponding detection point K on the X-ray detecting plane of the X-ray detector 52. The reconstruction process carried out based on X-ray image pixel data acquired in this state of displacement will result in errors due to the inclination of the axis of X-ray emission XA.
As a result, artifacts will appear in a final X-ray sectional image.
In order to perform a proper reconstruction process even when the axis of X-ray emission XA is inclined relative to the axis of revolution RA as described above, a different, sophisticated algorithm for reconstruction is used which incorporates complicated computing steps to cope with the inclination of the axis of X-ray emission XA. Alternatively, a reconstruction process is carried out by performing a data conversion according to an amount of inclination of the axis of X-ray emission XA for all of the X-ray image detection data outputted from the X-ray detector 52.
In such cases, however, the sophisticated algorithm incorporating complicated computing steps, or the data conversion performed beforehand for all of the X-ray image detection data, inevitably results in a sharp increase in the load of data processing.
Where the above algorithm is used, X-ray image detection data collected with the detection point K inclined relative to the direction of arrangement because of the displacement between the lattice point J and the detection point K on the X-ray detector as shown in FIG. 3 is not stored as data continuous along a row or column from each lattice point J as shown in FIG. 2. Instead, the data is stored as data arranged intermittently, skipping the row or column of the detection point K. Therefore, the X-ray image detection data stored intermittently is read intermittently little by little. This results in an inconvenience of not enabling an efficient calculating process to be performed by reading data to be used continuously at a time and temporarily storing the data in a cache memory providing a high-speed data transfer.